Washington (CNN)President Donald Trump on Wednesday fired Attorney General Jeff Sessions.

"At your request I am submitting my resignation," Sessions wrote in a letter to White House chief of staff John Kelly.

READ: Jeff Sessions' resignation letterMatthew Whitaker will take over as acting attorney general, the President said.Whitaker is expected to take charge of the Russia investigation and special counsel Robert Mueller from Deputy Attorney General Rod Rosenstein. Whitaker has been openly critical of Mueller and the investigation and Democrats immediately called on him to recuse himself, just as Sessions had.RELATED: Trump's replacement for Sessions once argued the Mueller probe goes too far."We are pleased to announce that Matthew G. Whitaker, Chief of Staff to Attorney General Jeff Sessions at the Department of Justice, will become our new Acting Attorney General of the United States. He will serve our Country well ...We thank Attorney General Jeff Sessions for his service, and wish him well! A permanent replacement will be nominated at a later date," Trump tweeted.The move is an abrupt end to what had been a tumultuous tenure for Sessions, originally one of Trump's earliest and most loyal surrogates as an Alabama Republican senator. He was a key figure in implementing Trump's vision for America and significantly rolled back Obama-era policies on immigration, police reform and civil rights.Sessions was an enforcer of much of the Trump administration's hardline approach on immigration and regularly praised the President's tough words on crime. But even as he continued to carry out the Trump agenda, his relationship with the President remained strained and fraught for months due to the ongoing Mueller investigation.Sessions received the request to resign from Kelly, not the President, on Wednesday morning, an administration official said. It is not clear whether Mueller was told ahead of time.Sessions believed that Rosenstein has handled the investigation properly after it was dropped "right in his lap," according to a source familiar with Sessions' thinking."[Rosenstein is] a professional, he's tried to do the right thing and he's handled it as well as anybody could," the source said of Sessions' views on the matter.However, the source said that Sessions himself has been frustrated that Mueller's investigation has not yet concluded, but DOJ officials have "tried to do the right thing every day and not be involved in arguing the case in the media."In a statement, Whitaker said he will lead a "fair" department with high ethical standards."It is a true honor that the President has confidence in my ability to lead the Department of Justice as Acting Attorney General. I am committed to leading a fair Department with the highest ethical standards, that upholds the rule of law, and seeks justice for all Americans," he said.

Trump constantly criticized SessionsSessions' ouster came a day after the midterm elections saw Republicans hold onto control of the Senate -- which would confirm Trump's eventual permanent choice to head the Justice Department -- and just weeks after Trump's former personal attorney Michael Cohen pleaded guilty to multiple counts of campaign finance violations, tax fraud and bank fraud and Trump's former campaign chair Paul Manafort was found guilty of eight charges including tax fraud and bank fraud.RELATED: Trump must reckon with new realities in wake of the electionSessions was aware that Cohen was facing bank fraud and tax violations but had been walled off from the campaign finance aspects of the investigation into Trump's former lawyer, a source with direct knowledge of the matter told CNN.Trump's distaste for Sessions was well known -- and publicly reinforced by the President himself on a regular basis -- after the attorney general recused himself from all matters related to the 2016 campaign early in Trump's term.